The present invention relates to the field of inspection of semiconductor devices. More particularly, the invention involves detection of contact failures such as not-open contact holes using a scanning electron microscope.
Integrated circuits are manufactured by first forming discrete semiconductor devices within silicon wafers. A multi-level metallic interconnection network is then formed in the devices contacting their active elements and connecting them together to create the desired circuits. The interconnection layers are formed by depositing an insulating layer over the discrete devices, patterning and etching contact openings into this layer, and then depositing conductive material into the openings. A conductive layer is then typically applied over the insulating layer. The conductive layer is then patterned and etched to form interconnections between the device contacts to create a first level of circuitry. Deposition of an insulating layer, formation of contact holes or via holes, formation of conductive material layers, and patterning, etc., are repeatedly carried out to create multi-level circuitry.
Depending upon the complexity of the overall integrated circuit, many levels, e.g., two to four levels, of metal are typically required to form the necessary interconnections and to connect the interconnections to contact pads which allow for the external connections to the completed circuit. A high density of integrated circuits designed to sub-micron dimensions requires extremely precise dimensional control and highly sensitive inspection methods to inspect the pattern of interconnections and/or the contact holes to assure the dimensional and structural integrity of the design patterns. These requirements are becoming more strict as circuits become more dense and miniaturized, such as with the mass-production of semiconductor memory devices such as 64M DRAM or 256M DRAM, which presently can typically require circuitry dimensions of 0.25 to 0.30 xcexcm.
Inspection of contact holes for conditions such as not-open conditions is becoming increasingly important because the aspect ratio (A/R) of a contact hole, i.e., the ratio of its depth to its diameter, has increased with the increasing demand for high density in semiconductor devices. However, normal optical microscopy using 488 nm wavelength visible light has a technical limitation in inspecting the inner features of contact holes because it does not permit a high enough degree of resolution to inspect inside features of the contact holes, which can be on the order of 200 nm or less in size. Optical microscopy is also not capable of providing beam spot size of 1 xcexcm or less.
An object of the present invention is to provide a contact failure inspection method and apparatus for semiconductor devices which provides for precise contact failure inspection for contact images by means of digitized values, not via naked eyes or microscope, which substantially obviates one or more problems due to the limitations and the disadvantages of the related art.
Another object of the present invention is to provide a contact failure inspection method for semiconductor devices and a contact failure inspection system for detecting the presence of contact failures for contacts having high aspect ratio, i.e., the ratio of a contact hole depth to its diameter.
Still another object of the present invention is to provide a contact failure inspection method for semiconductor devices and a contact failure inspection system for detecting contact failures on a wafer surface in a short time so as to be applied in mass production settings.
A further object of the present invention is to provide a method of manufacturing semiconductor devices using a contact failure inspection method and contact failure inspection system.
Another object of the present invention is to provide a contact failure inspection method and system for quickly detecting the location of contact failures to improve the production yield of semiconductor devices.
Another object of the present invention is to provide an inspection method and an inspection system for detecting the presence of pattern failures in semiconductor devices as well as photoresist pattern failures after a development processing during a photolithography process.
To achieve these and other objects, the present invention is directed to a method and apparatus for inspecting at least a portion of a semiconductor wafer. In the invention, scanning electron microscope (SEM) image data for the portion of the semiconductor wafer are read. Within the SEM image data, image data for a feature on the wafer are identified. A parameter related to the feature is computed and compared to a range of acceptable values for the parameter. Based on the comparison between the parameter and the range of acceptable values, the feature can be classified.
In one embodiment, the computed parameter is the dimension or size of the feature. For example, where the feature is a contact hole in an integrated circuit, the parameter may be the diameter of the hole measured in image data pixels. For example, a particular contact hole may be determined to be twenty pixels wide. In another embodiment, the parameter can be an average pixel intensity for pixels that are within the feature. Again, for example, where the feature is a contact hole, the parameter can be the average of the pixel intensities for the pixels that arc associated with the contact hole. Where the measured parameter is within the range of acceptable values for the parameter, the feature can be classified as acceptable. Where the parameter is outside the range of acceptable values for the parameter, the feature can be classified as a failure. For example, where the feature is a contact hole, the hole can be concluded to be a failure because, for example, it is not open.
In one embodiment of the invention, two parameters are calculated for the feature. The two parameters can be, for example, a dimension of a feature such as a contact hole, measured in pixels associated with the feature. The second parameter can be the average of the pixel intensities for the pixels associated with the feature. Both parameters are compared with predetermined ranges of acceptable values for the parameters. In one embodiment, where both parameters are simultaneously within their respective acceptable ranges, the feature, e.g., contact hole, can be classified as being acceptable. For example, a contact hole under these circumstances can be classified as open and properly sized and shaped. The relationship between the parameters and their respective ranges can be used to classify the feature as belonging to one of several types or categories. For example, each of the parameters can be used to classify a feature based on whether the parameter is below, within or above its acceptable range of values.
In one embodiment, the SEM image data are generated from both secondary electrons and higher-energy backscattered electrons in the scanning electron microscope. The data values are digitized and can be in the form of digitized grey scale pixel levels or color coded pixel values.
In one embodiment of the invention, a grid or mesh structure is used to characterize the features being inspected, such as by determining location and/or size of features being inspected. The grid or mesh structure typically includes a pair of mutually orthogonal axes superimposed over the image of the portion of the wafer being analyzed. Alternatively, the mesh axes can form any other appropriate geometric relationship, e.g., triangular, trapezoidal, etc. In one embodiment, the mesh location procedure determines location, shape and/or periodic patterns of the features by analyzing pixel values along a line parallel to one of the orthogonal axes which is successively positioned at pixel locations along the other orthogonal axis. For example, the mesh approach may include positioning a vertical line at multiple horizontal pixel positions and adding the vertical pixel intensity values at each horizontal position. The summed intensities can be compared at each horizontal position to identify an increase in intensity which can be used to indicate the presence of a feature such as a contact hole. This process can be repeated for a plurality of pixel positions along a single dimension. It can then be repeated in the orthogonal dimension such that the pattern, shape and size of all of the features can be determined.
This approach can also be used to determine the optimal size of a sub-grid or mesh unit containing features to be analyzed. For example, the mesh procedure can be used to select the optimal size of a mesh unit in pixels containing one hundred contact holes to be analyzed at a time. This approach makes the processing of the invention for inspecting features much more efficient in that unnecessary processing can be eliminated by optimizing the area of each region to be inspected.
In one embodiment, the SEM image pixel data arc used to compute an intensity profile for each feature, i.e., contact hole, being inspected. In one embodiment, the intensity profile is first generated by summing pixel intensity values for a feature along one orthogonal axis at each of a plurality of pixel positions disposed along the orthogonal rectangular axis. For example, at each horizontal pixel position, the pixel intensity values in the vertical direction are summed, averaged and plotted versus the horizontal axis pixel position. The pixel intensity profile can then be used to classify the feature in accordance with the invention.
In one embodiment, to normalize the intensity profiles for all of the features, the background intensity value is subtracted from all of the intensity values in each mesh unit. This has the effect of lowering the background value of each intensity profile to zero. Next, a threshold can be set in the normalized profile such that pixel intensities above the threshold are concluded to be associated with the feature being inspected. Next, the first and second parameters identified above can be computed from the profile. For example, the dimension of the feature can be calculated by counting the number of pixel positions along a first dimension which have summed intensity values in the orthogonal dimension that exceed the threshold. Since it is assumed that pixel intensity sums that exceed the threshold are associated with the feature, then the number of pixel positions having sums that exceed the threshold gives a measurement of a dimension of the feature, measured in pixels. The second parameter can be computed by computing an average of the intensity values that exceed the threshold. These two parameters can be compared to their respective predetermined ranges of acceptable values in order to classify the particular feature as belonging to one of the predetermined feature type classifications.
The inspection method and system of the invention provide numerous advantages over prior approaches. For example, certain prior approaches use optical methods such as optical microscopes or naked-eye examination to detect contact failures. These systems are unable to resolve small irregularities in features that result in failed circuits. The scanning electron microscope used in the connection with the present invention provides far superior resolution such that smaller irregularities can be detected. The invention is therefore applicable to present circuit features whose sizes are in the sub-micron range. Also, because of the mesh approach of the invention, the pixel data processing of the invention is extremely efficient. Processing and failure identification can be performed very efficiently and quickly such that the inspection method and system of the invention are highly applicable to wafer and circuit mass production settings.
In another aspect, the present invention is directed to a contact failure inspection method for semiconductor devices which comprises the steps of setting a processing cassette mounted with wafers having a plurality of contact holes formed on its surface; picking out a specific wafer from the cassette and loading it onto a stage inside a reference chamber of the SEM; aligning the loaded wafer for electron beam scanning; moving the stage mounted with the wafer to a specific position related to an incidence direction of electron beam of the SEM; opening a shutter for scanning electron beam onto a specific position of the wafer; auto-addressing for detecting inspection position by recognizing a pre-patterned reference image formed on the wafer; scanning the electron beam of the SEM onto the inspection position; auto-focusing for obtaining a further clear image by repeating the electron beam scanning; closing the shutter for isolating the auto-focused wafer from the electron beam; inspecting a contact failure by comparing the electron signal value detected from a unit surface containing at least one contact hole after scanning electron beam with an electron signal value defining a normal contact; further inspecting a contact failure in other position of the wafer by moving the stage to other position and repeating the same steps; and further inspecting a contact failure for all the wafers inside the cassette by unloading the finished wafer and loading other wafers into the reference chamber and repeating the same steps.
According to another aspect of the present invention, a method of manufacturing semiconductor devices comprises the steps of forming contact holes for specific insulating material layers formed on a semiconductor substrate; inspecting the contact of each contact hole by comparing the electron signal value detected from the surface including at least one contact hole with an electron signal value corresponding to a normal contact; and carrying out the subsequent processing for semiconductor devices fabrication process after charging conductive material layers inside the contact holes after the inspection.
The contact failure inspection step can be carried out for a specific sampling location on the semiconductor substrate, for example, to apply the contact failure inspection step to the mass-production line. After completing the development processing for the photoresist pattern formation, the failure inspection step can be further carried out for the bottom of the photoresist pattern for contact hole formation.
According to yet another aspect, the present invention includes a method of manufacturing semiconductor devices which comprises the steps of forming a photoresist contact hole pattern in order to form contact holes for insulating material layers formed on a semiconductor substrate; and inspecting the contact of each contact hole by comparing the electron signal value detected from a unit area including at least one contact hole pattern with an electron signal value corresponding to normal contact pattern.